A post-processing apparatus is used widely that carries out a post processing on a sheet on which an image is formed by an image forming apparatus. The post processing carried out by the post-processing apparatus includes, for example, a process of sorting sheets, a process of punching a hole on a sheet and a process of stapling a bunch of piled sheets. Operating speeds of the image forming apparatus and the post-processing apparatus are both improved, and thus less time is spent in forming an image and processing a sheet to improve the convenience. On the other hand, in the image forming apparatus or the post-processing apparatus, if a sheet jams, a user has to remove the jammed sheet and a sheet being processed, which is troublesome.